Animal by-products generated from poultry, ovine, bovine, swine and fish sources having limited market value as human food, typically, heads, feet, bone, horn, feathers, skin, chicken necks and backs, fish frames, certain whole fish and internal organs are cooked in a rendering process to remove both moisture content and fat to create a feed meal suitable for animals. Due to the relative high content of inorganic, non-combustible materials (ash) found in such feed meal, its direct use as a pet food is limited. It is known to those in the field of pet nutrition that excess amounts of minerals, such as calcium and phosphorus in the ash, is detrimental to both canine and feline health. The low ash, high protein fraction which is separated from rendered animal meals is commercially viable as both a canine and a feline feed, when the ash content is less than 11% by weight. Typically, untreated commercially rendered animal meals have an ash content greater than 11% and as much as 28% by weight.
Historically, a series of vibrating screens have been used to separate the denser high ash fraction from the less dense more protein ladened low ash fraction of rendered animal meals. This method has a number of disadvantages. For example, the screens used for the separation classify only by size and as a significant percentage of the high ash particles are close in size to the low ash particles, effective screen separation is difficult. Also, high ash bone splinters having narrow cross-sections tend to pass through the screens, with the desired low ash particles. In practice, vibrating screening of rendered animal meals normally provides a relatively low yield, e.g., 10 to 20% by weight of low ash material out of the rendered animal meal. Further, vibrating screens are relatively costly, energy intensive and frequently require recycling through several screens of varying size to provide their low yield of low ash material.
Another method known to the art for recovering the low ash fraction of rendered animal meal is discussed in U.S. Pat. No. 4,759,943. This patent teaches an air separation method wherein a strewing plate imparts tangential motion to the infeed stream of rendered animal meal, proximate to the streams entry of into the air classifier. An internal fan positioned coaxial to and mounted directly below the strewing plate, creates a counter-current air stream which entrains and carries away the desired lighter, lower density, low ash containing fines, from the more heavy, denser, undesired high ash fraction, to effect the separation. The denser high ash fraction is entrained in an air stream at the periphery of the air separator, where it is carried by the momentum imparted by the strewing plate, and wherefrom it falls by gravity to a first exit. The lower ash segment entrained in the counter-current air stream is directed to an internal channel, from which it is discharged from a second exit. The air separation method disclosed in U.S. Pat. No. 4,759,943, although more energy efficient than the vibrating screen separation method hereinbefore discussed, is still limited from a yield perspective, yields of about 33% by weight of the low ash fraction from the animal meal infeed being typical.
The present invention recovers the low ash component from rendered animal meals by using an adaptation of a dynamic air cyclone separator to avoid the drawbacks of the prior art hereinbefore discussed. The dynamic air cyclone separator has been used by the prior art to separate valuable minerals from waste gangue. Such mineral separation involves the classification of high density media that had been processed into fine 0.04 to 0.0005 meter particles (1.5 inch diameter to 28 mesh). In use, air is introduced tangentially into the separator housing at velocities of approximately 30 meters/second. The separator housing is formed of an upper cylindrical chamber provided with an upper air outlet and a lower conical section with a bottom material outlet. The rapid tangential inflow of the air creates a double vortex cyclone. The double vortex cyclone includes a first axially downward spiraling air flow along the outer walls of the cylindrical and conical sections to the lower outlet. Simultaneously, a second air flow spirals axially upward through the housing's center to the upper air outlet, the second air flow having a narrow diameter typically about 0.4 times that of the upper air outlet.
Cyclone separators have also been used for the separation of vegetable meals. The density differential between the desired protein fractions and non-protein fractions in vegetable meals is quite large, whereby a fraction having double the protein content of the untreated, infeed vegetable meal, mixture can be isolated. This method of air classification can be used to increase the protein content of meals made from such vegetable by-products as wheat flour, bean powders, and seed kernels. In this method, the mineral particles or vegetable meal is subjected to two opposing forces in the radial direction, laterally toward an outer wall of the separator. The first force is the centrifugal force of the downward vortex, which tends to throw the particle toward the outer wall and therefrom downward to be discharged through a bottom outlet. The second force is the drag of the air and eddy currents which tend to carry the particle to the central, axially upwardly, moving central spiral of air, whereby it is discharged through an upper air outlet. The movement of the particle, outwardly and down or inwardly and up, depends on the mass, density, configuration and size of the particle being acted on, as well as, the configuration of the separator, and the infeed vector and velocity of the air; all elements defining the tangential, radial and axial components of the velocity vectors acting on the particle. The degree of separation achieved by such prior art cyclone separators is primarily a function of the difference in particle size; such separators can be effective when substantial differences exist in the size and densities of the materials being classified.
For example, U.S. Pat. No. 4,257,880 discloses a primary cyclone separator which includes an upper cylindrical section and a lower conical section. The upper cylindrical section having a spinning vertical blade rotary rejector suspended from its top. Classifying air ladened with generally smaller, less dense, particles that have passed through the rotary rejector and exited from an upper air outlet in the primary cyclone separator are directed to a secondary cyclone separator. This secondary cyclone separator classifies the smaller, less dense, particles from the entraining air from which they are recovered. A fan loop recycles the air, from which the particles have been separated, from the secondary cyclone separator, at superatmospheric pressure back to the lower conical section of the primary air cyclone separator.
U.S. Pat. No. 4,963,634, discloses the use of a cyclone separator of the type disclosed in U.S. Pat. No. 4,257,880 for the preparation of polyvinyl chloride resins substantially free of fine-sized particles. U.S. Pat. No. 4,963,634 discloses that the degree of separation possible using the cyclone air separator of U.S. Pat. No. 4,257,880 is primarily governed by the air flow rate into the primary cyclone separator, the rotational speed of the vertical blade rotary rejector and the infeed rate of raw material into the primary cyclone separator. U.S. Pat. No. 4,963,634 discloses an air flow into the primary cyclone separator at a fan rotational speed of 3,900 RPM (revolutions per minute) and a rotor rotational speed of approximately 900 RPM to effect the desired separation of fine-sized polyvinyl chloride particles.
The use of dynamic air cyclone separators, as disclosed in U.S. Pat. No. 4,963,634, is not directly applicable to the segregation of rendered animal meals. The components of animal feed meals made from rendered animal by-products do not have the wide disparity of size and density as is present in the mineral, vegetable meal and plastic resin infeeds, for which these separators have been used by the prior art. For example, when it was attempted to use the cyclone separator disclosed in U.S. Pat. No. 4,257,880 for the recovery of a low ash fraction from rendered animal meals, following the teachings of U.S. Pat. No. 4,963,634, disappointingly low yields of low ash material were obtained. Specifically, low ash yields of from 25 to 32% by weight were obtained from a rendered animal by-product infeed at an air inflow fan RPM of about 3,100, and at rotary rejector speeds of about 700 RPM.
Accordingly, there is need for an improved process to separate the low ash fraction of rendered animal meals over the prior art air separation means, which does not suffer from low yield and is accordingly more economical and efficient.